You can also simply change the filename do a
.txt extension. Refer to this answer:
There are 3 possible extensions which doxygen treats as additional documentation files: .dox, .txt, and .doc.
Files [with] such extension are hidden from the file index. Inside the file you need to put one or more C/C++ style comment blocks.
So rename 'documentation.cs' to 'documentation.dox', but leave the C-style comments inside. (assuming that's what you use - not sure why you decided to add the .cs extension, it doesn't seem like it's actually a C# source file)
Make sure you leave the file in the current directory (if INPUT is left empty) or add it to a directory included in the
EDIT: According to this webpage you may have to add
FILE_PATTERNS += *.dox to your doxyfile. I don't know if the file extensions are automatically supported nowadays or not but they weren't when the blog was written.